1. Field of the Invention
The present invention relates to isolation of sensitive circuitry from noisy circuitry and, in particular, to isolation of digital circuit sections having noise from sensitive analog circuit sections within the same integrated circuit.
2. Description of the Related Art
Noise is problem that manifests itself in a variety of ways. One manifestation occurs when a circuit or circuit section produces noise that can adversely interfere with neighboring circuits. The neighboring circuits may be of different types, and may be mounted in a common housing or substrate. For example, so-called "mixed signal" integrated circuits can provide more than one type of circuit in various areas on a common substrate. The term integrated circuit (IC) may be used in this specification to refer to the actual physical substrate itself as well as the circuits formed therein.
The above-mentioned problem can be particularly acute in ICs having both sensitive analog sections and digital circuit sections. For example, an IC having RF (radio frequency) components may contain both a digital signal processing portion as well as an analog RF receiver portion, which may be configured to receive unbalanced analog signals in the microvolt range. The digital portion generates high frequencies and harmonics and other noise due, in part, to the sharp edges of the digital waveforms used for clock signals and the like. This digital noise can be communicated to the sensitive analog RF receiver portion through the common substrate and can adversely affect the operation of the RF receiver. These adverse effects are exacerbated when the input analog signals are unbalanced rather than balanced. Such noise or related types of noise are sometimes referred to as cross-coupling or cross-talk.
Various techniques have been proposed and utilized to attempt to isolate noisy digital circuit sections from sensitive analog circuit sections within the same IC. Such techniques include isolation by physical separation; isolation by separate supply rails; isolation by grounded guard rings/substrate trenches; differential circuitry in the sensitive analog portion; use of low noise injecting digital circuitry, such as differential logic cells (emitter coupled logic style); and making the digital circuitry portions synchronous with the analog function and moving the clock edges away from critical analog sampling instances.
Such conventional techniques, however, are not always sufficient or fully effective, or practical, feasible, or cost-effective to implement.